On Fri, Aug 16, 2024 at 02:10:36PM +0800, Jijie Shao wrote:

on 2024/8/16 10:25, Andrew Lunn wrote:

quoted

quoted

+struct hbg_mdio_command {
+	union {
+		struct {
+			u32 mdio_devad : 5;
+			u32 mdio_prtad :5;
+			u32 mdio_op : 2;
+			u32 mdio_st : 2;
+			u32 mdio_start : 1;
+			u32 mdio_clk_sel : 1;
+			u32 mdio_auto_scan : 1;
+			u32 mdio_clk_sel_exp : 1;
+			u32 rev : 14;
+		};
+		u32 bits;
+	};
+};

This is generally not the way to do this. Please look at the macros in
include/linux/bitfield.h. FIELD_PREP, GENMASK, BIT, FIELD_GET
etc. These are guaranteed to work for both big and little endian, and
you avoid issues where the compiler decides to add padding in your
bitfields.

	Andrew

Thanks, I already know about macros like FIELD_PREP or FIELD_GET.
and these macros are already used in parts of this patch set.

But I think this writing style in here very convenient.
Although padding needs to be added during definition,
but you can use command.mdio_start or command->mdio_start
to access specific bitfields.

Although FIELD_PREP/FIELD_GET is convenient,
But it also needs to define the mask using BIT or GENMASK,
and the mask can only be a static constant,
which makes it difficult to use sometimes.

Have a look around. How many drivers use this sort of union? How many
use bitfield.h. There is a reason the union is not used. I suspect
there is nothing in the C standard which guarantees it works.

      Andrew